Method and apparatus for real time identification and correction of pixel defects for image sensor arrays

ABSTRACT

An image processing system and method compares each pixel of an image obtained from an image sensor array with at least eight surrounding pixels of the same color in the filter array. If the signal of a given pixel is larger than the respective signals of all eight surrounding pixels of the same color, then the value of that central pixel signal is substituted with the maximum signal value among the surrounding eight pixels of the same color. Similarly, if the signal of a given pixel is smaller than the respective signals of all eight surrounding pixels of the same color, then the value of that central pixel signal is substituted with the minimum signal value among the surrounding eight pixels of the same color.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus that enablesreal time identification and correction of defective pixels in an imagesensor array in a digital imaging environment.

BACKGROUND OF THE INVENTION

When an image is exposed onto an image sensor, each pixel records theamount of light that it “sees” as an intensity level between a darksignal wherein no light reaches that pixel, to a full white signalrepresenting the maximum amount of light detectable by that pixel. Theimage thus captured by the image sensor is processed as a grayscaleimage.

To detect the colors of the image exposed onto the image sensor, thepixels of the image sensor are covered with a respective color filterthat absorbs light wavelengths for all colors except the color of thefilter. An exemplary method for acquiring color information from animage sensor is to place a color filter array over the pixels of animage sensor. The most common example of such a color filter array is aBayer mosaic filter, shown in FIG. 1. The Bayer mosaic filter has acheckerboard like configuration and is composed of alternating rows ofred and green, and blue and green filters. The red and blue filters areoffset from each other so that no two green filters share an edgebetween adjacent rows and columns. To obtain complete color informationfor each pixel, it is necessary to interpolate the intensity of thecolors based on the level of those colors at the surrounding pixels.

Although a typical image sensor has at least hundreds of thousands ofpixels collecting color filtered information for an image, each pixel isimportant, not just for the signal value recorded in that pixel, butalso for use in interpolating color information for other surroundingpixels. Thus, when a pixel is defective, its effects can be compoundedto affect a significant portion of the image.

Due to a number of inherent variabilities in the manufacturing processesof image sensors such as charge-coupled devices (CCDs) or complementarymetal oxide semiconductors (CMOSs), some of the pixels of the imagingarray in each sensor are either always dark (often due to a short in thecircuitry) or always too bright (often due to abnormally high leakagecurrent). In most cases these defects can be corrected by substitutingthe defective signal values with the values of adjacent pixels duringimage processing. However, this substitution requires knowledge of thedefective pixel locations.

In most cameras presently known, the locations of the defective pixelsare determined during an off-line testing procedure during theproduction stage and are stored in a non-volatile memory in the camera.The main drawback of this conventional approach is that the number ofdefects that can be corrected is limited by the size of the non-volatilememory dedicated to this purpose. Another drawback of the conventionalapproach is that it requires a separate manufacturing step for theidentification and storage of the defect locations.

Other existing camera modules perform correction of defective pixels bycomparing each pixel with adjacent pixels, such as those on either sideof the pixel in the same line, and performing substitution usingthreshold-based criteria. Such defect correction methods usingcomparisons based on predetermined thresholds tend to diminishresolution, as fine details such as thin vertical lines are “eliminated”from the image.

In view of the present state of the art, the process for correctingdefective pixels would thus be greatly improved if the dedicatednon-volatile memory currently required for storing the pre-calibrateddefect map could be eliminated, and if the correction of defectivepixels could be made without using threshold criteria.

BRIEF SUMMARY OF THE INVENTION

The present invention addresses the disadvantages of the prior art byproviding a method and apparatus which enables real time correction ofimage data for an arbitrary number of both dark and bright defectivepixels in an image sensor array without the need for a separateproduction-stage calibration to account for the defective pixels or anon-volatile medium to store a pre-calibrated defect map. The method andapparatus of the invention also enables the identification of defectivepixel data without relying on a specific definition of a defect andwithout requiring the use of any specified thresholds against whichpotential defects are compared. The method and apparatus of theinvention also enables correction of defective pixel data in an imageregardless of the image contents and the correction of defective pixeldata in an image without appreciably affecting the resolution of theimage, and the correction of defective pixel data in an image, whilealso reducing peak-to-peak noise variations in the image.

An image processing system and method of the invention compares thesignal of each pixel in an image with the respective signals of at leasteight surrounding pixels of the same color in the filter array. If thesignal of a given pixel is larger than the respective signals of alleight surrounding pixels of the same color, then the value of thatcentral pixel signal is substituted with the maximum signal value amongthe surrounding eight pixels of the same color. Similarly, if the signalof a given pixel is smaller than the respective signals of all eightsurrounding pixels of the same color, then the value of that centralpixel signal is substituted with the minimum signal value among thesurrounding eight pixels of the same color.

The present invention also includes the capability to detect a clusterof defective pixels in an image sensor pixel array, which may beperformed after the defective pixel data identification and correctionoperation, and in which the absolute value of the difference between thesignal values of two adjacent pixels of the same color type are comparedagainst a threshold value, wherein a cluster defect is present if theabsolute value of any difference between two signal values is determinedto be greater than the threshold. Preferably, this operation is onlyperformed during testing at the factory, but may also be configured toexecute automatically in the imaging apparatus after completion of thedefective pixel data identification and correction operation.

Other features and advantages of the present invention will becomeapparent from the following description of the invention with referenceto the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a known Bayer mosaic filter pattern used onan image sensor;

FIG. 2 is a diagram illustrating a method for identifying and correctingdefective pixel data in accordance with the present invention;

FIG. 3 is a diagram illustrating a method for detecting a cluster ofdefective pixels in an image sensor pixel array in accordance with thepresent invention;

FIG. 4 is a diagram illustrating a processing unit implementing thedefective pixel data identification and correction method of the presentinvention;

FIG. 5 is an illustration of an imaging apparatus incorporating thedefective pixel data identification and correction system and method ofthe present invention; and

FIG. 6 is an illustration of a processing system communicating with animaging apparatus of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the method aspect of the present invention, the signal value of eachpixel in an image obtained by an image sensor is compared with thevalues for at least eight closest surrounding pixels having the samecolor filter located adjacent to or near the pixel being tested. Anexample of this method will be described for an image obtained by animage sensor having a Bayer mosaic filter array. As illustrated in FIG.2, the signal value of the central pixel, P₀, is compared with thesignal values for each of the eight closest surrounding pixels of thesame color. In this example, these eight closest surrounding pixels arelocated along the same row (P₄, P₅), column (P₂, P₇), and diagonally(P₁, P₃, P₆, and P₈) from the pixel being tested (P₀), each spaced onepixel away from P₀ in the relevant direction.

If the signal of P₀ is larger than the respective signals of all eightsurrounding pixels P₁ through P₈, then the signal value for P₀ issubstituted with the maximum signal value from among the signal valuesfor pixels P₁ through P₈. Similarly, if the signal of P₀ is smaller thanthe respective signals of all eight surrounding pixels P₁ through P₈,then the signal value P₀ is substituted with the minimum signal valuefrom among the signal values for pixels P₁ through P₈.

Stated in mathematical terms, if P₀=max(P₁, . . . , P₈) or if P₀=min(P₁,. . . , P₈), then P₀=P₁, where P_(i) satisfies |P₀−P_(k)|=min|P₀−P_(k)|,wherein k=1, . . . , 8.

Since this method requires image data from two rows and columns on eachside of a pixel being tested (one on each side for monochromaticfiltered arrays), the method as described above does not account forchecking and correcting, if necessary, the pixels in the two outermostrows and columns of the image obtained by the image sensor becausepixels in these locations do not have two rows and columns ofsurrounding pixels. One solution is to provide a slightly larger pixelarray in the image sensor than will be displayed or outputted from theapparatus in which the image processing system of the present inventionis incorporated. Hence, the image data contained in these pixels fromthe one or two rows and columns at the edges of the image sensor pixelarray are not checked for defects and are not displayed, but are usedfor checking the function of the relevant interior pixels according tothe present invention.

The method described above is preferably performed automatically in animaging apparatus, such as a camera, for each image captured by theimaging sensor provided in the apparatus, and is effective for 8-bit,10-bit, 12- bit, etc. bit depths of pixel color for each color channelin the color filter array over an image sensor. The operation of thesemethod is transparent to an end user of the apparatus, so that the userwill only see the corrected image without the initial defects. Moreover,it is noted that if the imaging apparatus has passed the quality controltests performed at the factory and is being operated by the end user,the cluster defect detection method should always produce a negativeresult, indicating that no cluster defects are present.

However, this method is only suited to correct isolated defectsoccurring no more frequently than once per 3×3 or 5×5 or other subarraysize as needed for testing each pixel. In the case of a cluster ofdefective pixels, the method fails and the device should be rejected atthe production testing stage. A cluster is defined as two or moredefects of the same color located next to each other. In a Bayer mosaicfiltered array, a cluster would include two or more defective pixels ofthe same color which are spaced apart from each other by an interveningpixel of another color.

Optionally, the capability to perform the following operation may beprovided in the imaging apparatus to assist in determining the presenceof clusters after the performance of the defect correction methoddescribed above. In this optional operation, the image sensor isuniformly illuminated following the defective pixel identification andcorrection process, and as illustrated in FIG. 3, the absolute value ofthe difference between the values of two adjacent pixels of a same colorP₀₊ and P₆₊ is compared against a threshold value. Differences above thethreshold level will be observed only if uncorrectable cluster defectsare present in the image sensor. Stated mathematically, a cluster ofdefective pixels is present if, after execution of the defect correctionprocess,|P ₀₊ −P ₆₊ |>T _(th).

The cluster defect detection operation may be set to be performedautomatically in the imaging apparatus after a completion of thedefective pixel data identification and correction operation, but ispreferably controlled, such as by a switch in the system hardware, to beoperated at the production stage after a complete test execution of thedefective pixel data identification and correction operation. Whenperformed at the factory during a production stage or quality controlprocedure, a successful execution of this cluster detection test servesas a verification that the image sensor is free of cluster defects.However, if the test yields a result which is greater than the thresholdvalue, a cluster of defective pixels is present, and the image sensorshould be discarded.

The defective pixel data identification and correction method describedabove is implemented in a system which includes a memory structurecapable of holding image information obtained from at least a 3×3subset, and preferably a 5×5 subset, of a pixel array of an imagesensor. For example, if the architecture of the image sensor provides amonochrome image, such as by having a monochrome filter over the entirepixel array or by producing an image in grayscale, only three lines ofstorage are necessary for the memory frame because each pixel isimmediately surrounded by eight pixels of the same color as the one tobe tested. If, on the other hand, the image sensor architecture includesa multi-colored filter array, such as a Bayer mosaic filter in whichrows of red and green alternating pixels are alternatingly arranged withrows of blue and green alternating pixels as shown in FIG. 1, then thememory structure of the present invention is capable of holding at leastfive lines of image information. Alternatively, the memory structure maybe of any other size necessary for accommodating any other type of imagesensor filter arrangement. As a further alternative, the buffer memorymay be a full frame buffering memory capable of holding pixelinformation for an entire image.

FIG. 4 is a diagram illustrating a preferred embodiment of a processingunit 100 incorporated in an image processor and which implements thecorrection method described above. In the preferred embodiment, theprocessing unit 100 includes six rows of shift registers 108 forreceiving image data, a delay circuit 112 for transferring a most recentline of image data into one of the rows of shift registers 108 in columnsynchrony with input of data into other rows of shift registers, fivebanks of random access memory (RAM) 102 for storing an equal number ofpreviously read in lines of image data, a read/write address generator110 for controlling the flow of data into and out of the RAM banks 102,a input multiplexer 104 for inputting the image data into theappropriate RAM as determined by the read/write address generator 110,an output multiplexer 106 for outputting the image data from the RAMbanks 102 into the appropriate row of shift registers 108 as determinedby the read/write address generator 110, a defect correction circuit forperforming defective pixel data identification and correction asdescribed above, and optionally, a second two-dimensional interpolationimage processing operation circuit 116 for producing red, green and blueoutput signals.

For a monochrome image sensor array, since only three rows of shiftregisters are needed to perform the defective pixel data identificationand correction operation fewer RAM banks 102 and rows of shift registers108, e.g., three RAM banks 102 and four rows of shift registers 108, areneeded.

In the preferred embodiment, each memory bank is a dual-ported SRAM(static RAM) allowing simultaneous read/write access. However, othertypes of random access memories may be used, such as any of the manydifferent subspecies of DRAMs, including, for example, fast page modeDRAM (FPM DRAM), extended data out DRAM (EDO DRAM), burst EDO DRAM,synchronous DRAM (SDRAM), double data rate DRAM (DDR DRAM), Rambus DRAM(RDRAM), etc.

Additionally, the delay circuit can be embodied as a RAM or any otherdevice, or may be substituted with any other mechanism through which anincoming line of image data can be transferred directly into the shiftregisters in column synchrony with the image data entering the shiftregisters through the RAM banks 102. In the preferred embodiment,however the delay circuit 112 is simply provided as a delay registerwhich delays the input of a new line of image data into an upper row ofthe shift registers 108 to maintain pixel column synchronism with pixeldata input into the remaining rows of shift registers from the RAM banks102.

The six sets of shift registers 108 provide simultaneous access to sixlines of image data (fewer lines are needed for a monochrome imagearray) to enable both pixel defect correction and anothertwo-dimensional interpolation processing function to be performed whilesharing the same shift register hardware as used for defect correction.As mentioned above, the most recent line of image data is transferred tothe top row of shift registers shown in FIG. 4, while the next mostrecent line of image data is inputted into the second row of shiftregisters from one of the RAM banks 102, and so forth, with the sixthmost recent line of image data being inputted into the last row of shiftregisters shown in FIG. 4.

The particular RAM bank from which data is being transferred into eachrespective row of shift registers 108 is determined by a pointer inaddress generator 110 which operates a switch provided in the outputmultiplexer 106. The data in the RAM banks 102 are read out to the shiftregisters 108 pixel by pixel in FIFO order. Also, the image data fromthe delay register 112 is synchronized with the output from the memorybanks 102 so that the image data being fed into one set of the shiftregisters by the delay register is from the same column of the imagearray as the pixel information being read out of the five banks ofmemory into the other five sets of shift registers.

The architecture described above allows for defect correction to beperformed at the pixel clock rate prior to further processing steps inthe image processor. The identification and correction of defectivepixels occurs in the shift registers by the defect correction circuit114, and is sequentially performed for pixel data in the third row ofshift registers using the data read into the first five rows of shiftregisters. Specifically, pixel data from each line of image data in thememory banks 102 is parsed into the appropriate shift registers insynchronization with the pixel clock rate, and detection and correctionof data for defective pixels is performed for the central pixel 122shown in the defect correction circuit 114 of the shift register array.Once the pixel data currently in the center of the defect correctioncircuit 114 has been checked and corrected, if necessary, the data inthe shift register array shifts to the right, with new pixel data beingread into the left most column of shift registers from the respectiveRAM banks 102 and the delay register 112, to check the pixel data forthe next pixel to the left in the same line.

This process is repeated until all of the pixel data in the relevantimage line, i.e. the third row of shift registers, has been checked andcorrected, if necessary. As each line of image data is being movedthrough the shift registers, the RAM bank containing the oldest line ofimage data which is moving through the sixth row of shift registers isalso being loaded with the new line of pixel data entering the first rowof shift registers under control of the input multiplexer 104 andaddress generator 110. Thus, the old image data is overwritten as it isbeing read out to the sixth line of shift registers via the outputmultiplexer 106. As a result, the RAM bank reading out the oldest lineof image data is loaded with the newest line of image data currentlyentering the first row of shift register. This RAM will thus containpixel image data to be loaded into the second row of shift registers forcorrecting the next line of image data.

Upon reaching the end of the image data lines, the read/write addressgenerator 110 updates the output multiplexer 106 so that the RAM bank102 containing the line of image data just checked by the defectcorrection circuit 114 is rerouted to read out its contents to thefourth row of shift registers shown in FIG. 4, while the RAM bank 102previously outputting its image data to the fourth row of shiftregisters is now configured to output to the fifth row of shiftregisters. Similarly, the RAM bank previously configured to read out itsline of image data into the fifth row of shift registers is now reroutedto read out to the sixth row of shift registers, and the RAM bankpreviously outputting to the second row of shift registers is nowconfigured to output to the third row of shift registers. As explainedabove, the image data previously read into the first row of shiftregisters is now resident in the RAM bank 102 previously holding theoldest line of image data and is now routed to the second row of shiftregisters.

The image data in the RAM banks 102 is then read out to the shiftregisters from left to right, as in the previous iteration, to begin theprocess anew for the line of pixel data now present in the third row ofshift registers 108. Also, the next incoming line of image data is readinto the first row of shift registers in column synchrony with the imagedata entering the second through sixth rows of shift registers from theRAM banks 102. In the preferred embodiment, column synchronization ofthe image data being read into the first row of shift registers relativeto the remaining rows is achieved by a delay register 112 as shown inFIG. 4, although this may be achieved using any of the other meansmentioned above.

Since the data buffered in the memory banks is used repeatedly, i.e.once for each line in the five RAM banks (three for monochrome imagearrays), it is necessary to update the memory contents once a defect isfound and corrected. The corrected value is written into the centralshift register 122 in defect correction circuit 114 and also back intothe appropriate pixel location in the RAM bank 102, via the inputmultiplexer 104, corresponding to the row of shift registers on whichdefect correction is being made (120 in FIG. 4).

Once the defect correction is performed, the same set of shift-registerscan be used (with a delay of one line and one pixel) to perform othertwo-dimensional image processing operations such as color interpolation,indicated, for example, as circuit 116 in FIG. 4. Other image processingoperations such as sharpness filtering, white balancing, etc. can alsobe performed by circuit 116 to produce a red, green and blue outputsignal 124.

For a monochrome image sensor array, the operation of the processingunit 100 is the same as described above, except that fewer RAM banks androws of shift registers are provided, and the output signal 124 from thetwo-dimensional image processing circuit 116 will only have the color ofthe monochrome image.

An example of an imaging apparatus 200 incorporating the features of thepresent invention discussed above is shown in FIG. 5, and includes alens system 202 for directing light from an object to be imaged to theimage sensing unit 204 including an image sensor; an analog-to-digitalconverter 206 for converting the image signals received at the imagesensing unit 204 into digital signals; the image/color processing unit208 for performing image correction processes including the datacorrection for defective pixels as described above and also forperforming other processes such as color interpolation, sharpnessfiltering, white balancing, etc.; an output formatconversion/compression unit 210 for converting the image data into anappropriate file format for being outputted or displayed to the user;and a controller 212 for controlling the operations of the entireimaging apparatus 200.

The image sensor in the image sensing unit 204 is preferably constructedas an integrated circuit which includes pixels made of a photosensitivematerial such as silicon. The image sensor may be formed as a CMOSsensor and combined with a processor, such as a CPU, digital signalprocessor or microprocessor, in a single integrated circuit.Alternatively, the image sensor in the image sensing unit 204 may beconstructed as a charge coupled device (CCD).

Without being limiting, such an imaging apparatus 200 could be part of acomputer system, camera system, scanner, machine vision system, vehiclenavigation system, video telephone, surveillance system, auto focussystem, star tracker system, motion detection system, imagestabilization system and data compression system for high-definitiontelevision, all of which can utilize the present invention.

An exemplary processor system 400 to which the imaging apparatus 200 maybe connected is shown in FIG. 6. The processing system 400, such as acomputer system, for example, generally comprises a central processingunit (CPU) 444 that communicates with an input/output (I/O) device 446over a bus 452. The imaging apparatus 200 communicates with the systemover bus 452 or a ported connection. The processor system 400 alsoincludes random access memory (RAM) 448, and, in the case of a computersystem, may include peripheral devices such as a floppy disk drive 454and a compact disk (CD) ROM drive 456 which also communicate with CPU444 over the bus 452.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

1. An apparatus for correcting pixel image data, comprising: a pluralityof memory banks each for receiving and temporarily storing a line ofimage data of an image obtained from an image sensor array, each linecontaining signal value data respectively corresponding to a line ofpixels in the array; a plurality of sets of shift registers for eachmemory bank for receiving the image data from the plurality of memorybanks; a correction circuit for testing the signal value of a test pixelin the image data in one of the sets of shift registers, determining ifthe signal value of the test pixel is defective, and correcting thedefective signal value if found to be defective by comparing the signalvalue of a test pixel with the signal values of at least eight pixelssurrounding the test pixel in the image, and replacing the signal valueof the test pixel with one of either the largest signal value from amongthe eight surrounding pixels or the smallest signal value from among theeight surrounding pixels, leaving the signal values of the eightsurrounding pixels unchanged; and a delay register for inputting a lineof image data directly from the image sensor array into one of theplurality of sets of shift registers in column synchrony with the imagedata being inputted into the shift registers from the memory banks. 2.An apparatus for correcting pixel image data, comprising: a plurality ofmemory banks each for receiving and temporarily storing a line of imagedata of an image obtained from an image sensor array, each linecontaining signal value data respectively corresponding to a line ofpixels in the array; a plurality of sets of shift registers for eachmemory bank for receiving the image data from the plurality of memorybanks; a correction circuit for testing the signal value of a test pixelin the image data in one of the sets of shift registers, determining ifthe signal value of the test pixel is defective, and correcting thedefective signal value if found to be defective by comparing the signalvalue of a test pixel with the signal values of at least eight pixelssurrounding the test pixel in the image, and replacing the signal valueof the test pixel with one of either the largest signal value from amongthe eight surrounding pixels or the smallest signal value from among theeight surrounding pixels, leaving the signal values of the eightsurrounding pixels unchanged; and an image processing circuit forperforming an image processing operation on a pixel of the image dataalready tested, the image processing circuit using a portion of theshift registers which is offset from the shift registers in which thetest pixel has been tested.
 3. The apparatus of claim 2, wherein theimage processing circuit is further provided for performing a colorprocessing operation.
 4. The apparatus of claim 2, wherein the imageprocessing circuit is further provided for performing a colorinterpolation operation.
 5. The apparatus of claim 2, wherein the imageprocessing circuit is further provided for performing a white balancingoperation.
 6. The apparatus of claim 2, wherein the image processingcircuit is further provided for performing a sharpness filteringoperation.
 7. A method for correcting pixel image data, comprising: a)obtaining at least three lines of image data containing signal valuesfor a corresponding number of lines of an image obtained from a pixelarray in an image sensor; b) comparing a signal value of a test pixelfrom among the at least three lines of signal values with the respectivesignal values of at least eight pixels from among the at least threelines of signal values and which surround the test pixel, having theeight surrounding pixels the same color type as the test pixel; c) ifthe signal value of the test pixel is larger than all of the signalvalues for the eight surrounding pixels, redefining the signal value ofthe test pixel to be equal to the maximum signal value from among thesignal values of the eight surrounding pixels, leaving the signal valuesof the eight surrounding pixels unchanged; and d) if the signal value ofthe test pixel is smaller than all of the signal values for the eightsurrounding pixels, redefining the signal value of the test pixel to beequal to the minimum signal value from among the signal values of theeight surrounding pixels, leaving the signal values of the eightsurrounding pixels unchanged, wherein the comparing act is not performedfor the signal values of the pixels from the outermost two rows andcolumns in the image sensor pixel array.
 8. A method for correctingpixel image data, comprising: a) obtaining at least three lines of imagedata containing signal values for a corresponding number of lines of animage obtained from a pixel array in an image sensor; b) comparing asignal value of a test pixel from among the at least three lines ofsignal values with the respective signal values of at least eight pixelsfrom among the at least three lines of signal values and which surroundthe test pixel, having the eight surrounding pixels the same color typeas the test pixel; c) if the signal value of the test pixel is largerthan all of the signal values for the eight surrounding pixels,redefining the signal value of the test pixel to be equal to the maximumsignal value from among the signal values of the eight surroundingpixels, leaving the signal values of the eight surrounding pixelsunchanged; and d) if the signal value of the test pixel is smaller thanall of the signal values for the eight surrounding pixels, redefiningthe signal value of the test pixel to be equal to the minimum signalvalue from among the signal values of the eight surrounding pixels,leaving the signal values of the eight surrounding pixels unchanged; e)repeating acts a) through d) for a next pixel in the same line of imagedata as the tested pixel, until all the pixels in the line have beentested; f) obtaining a new line of signal values for an new line ofimage data from the image sensor pixel array; and g) repeating act e) ona next line of image data and also repeating act f) until all the signalvalues for each pixel in each line of image data have been tested; h)verifying that no cluster of defective pixels is present after testingeach pixel in each line of image data by; i) selecting a first pixelfrom among the tested image data; j) selecting a second pixel from amongthe closest surrounding pixels to the first pixel of the same color typeas the first pixel; k) obtaining a difference in signal values betweenthe first pixel and second pixel; l) comparing the absolute value of thedifference to a threshold value, wherein a cluster of defective pixelsis determined to be present if the absolute value of the difference isgreater than the threshold value; and m) repeating acts i) through l)for each pixel in the tested image data.
 9. A method for correctingpixel image data, comprising: writing at least three lines of image datacontaining signal values for a corresponding number of lines of an imageobtained from a pixel array in an image sensor, into a plurality ofmemory banks; sequentially inputting the image data from the memorybanks into a plurality of sets of shift registers; comparing a signalvalue of a test pixel in the shift registers with the signal values ofat least eight surrounding pixels in the shift registers and which areof a same color filter type as the test pixel; if the signal value ofthe test pixel is larger than all of the signal values of the eightsurrounding pixels, correcting the signal value of the test pixel to beequal to the largest signal value from among the eight surroundingpixels, leaving the signal values of the eight surrounding pixelsunchanged; if the signal value of the test pixel is smaller than all ofthe signal values of the eight surrounding pixels, correcting the signalvalue of the test pixel to be equal to the smallest signal value fromamong the eight surrounding pixels, leaving the signal values of theeight surrounding pixels unchanged; inputting a line of image datadirectly into a set of shift registers separately from the plurality ofmemory banks, but in column synchrony with the lines of image data beinginputted into the shift registers from the memory banks, wherein aportion the signal values of the at least eight surrounding pixels forbeing compared with the signal value of the test pixel are selected fromamong the image data inputted directly into the shift registers.
 10. Amethod for correcting pixel image data, comprising: writing at leastthree lines of image data containing signal values for a correspondingnumber of lines of an image obtained from a pixel array in an imagesensor, into a plurality of memory banks; sequentially inputting theimage data from the memory banks into a plurality of sets of shiftregisters; comparing a signal value of a test pixel in the shiftregisters with the signal values of at least eight surrounding pixels inthe shift registers and which are of a same color filter type as thetest pixel; if the signal value of the test pixel is larger than all ofthe signal values of the eight surrounding pixels, correcting the signalvalue of the test pixel to be equal to the largest signal value fromamong the eight surrounding pixels, leaving the signal values of theeight surrounding pixels unchanged; if the signal value of the testpixel is smaller than all of the signal values of the eight surroundingpixels, correcting the signal value of the test pixel to be equal to thesmallest signal value from among the eight surrounding pixels, leavingthe signal values of the eight surrounding pixels unchanged; and uponcorrecting the signal value of the test pixel, replacing the correctedvalue in the corresponding location in the corresponding memory bank.11. A method for correcting pixel image data, comprising: writing atleast three lines of image data containing signal values for acorresponding number of lines of an image obtained from a pixel array inan image sensor, into a plurality of memory banks; sequentiallyinputting the image data from the memory banks into a plurality of setsof shift registers; comparing a signal value of a test pixel in theshift registers with the signal values of at least eight surroundingpixels in the shift registers and which are of a same color filter typeas the test pixel; if the signal value of the test pixel is larger thanall of the signal values of the eight surrounding pixels, correcting thesignal value of the test pixel to be equal to the largest signal valuefrom among the eight surrounding pixels, leaving the signal values ofthe eight surrounding pixels unchanged; if the signal value of the testpixel is smaller than all of the signal values of the eight surroundingpixels, correcting the signal value of the test pixel to be equal to thesmallest signal value from among the eight surrounding pixels, leavingthe signal values of the eight surrounding pixels unchanged; uponcorrecting the signal value of the test pixel, replacing the correctedvalue in the corresponding location in the shift registers.
 12. A methodfor correcting pixel image data, comprising: writing at least threelines of image data containing signal values for a corresponding numberof lines of an image obtained from a pixel array in an image sensor,into a plurality of memory banks; sequentially inputting the image datafrom the memory banks into a plurality of sets of shift registers;comparing a signal value of a test pixel in the shift registers with thesignal values of at least eight surrounding pixels in the shiftregisters and which are of a same color filter type as the test pixel;if the signal value of the test pixel is larger than all of the signalvalues of the eight surrounding pixels, correcting the signal value ofthe test pixel to be equal to the largest signal value from among theeight surrounding pixels, leaving the signal values of the eightsurrounding pixels unchanged; if the signal value of the test pixel issmaller than all of the signal values of the eight surrounding pixels,correcting the signal value of the test pixel to be equal to thesmallest signal value from among the eight surrounding pixels, leavingthe signal values of the eight surrounding pixels unchanged; after thecomparing act and, if necessary, the correcting act have been performedfor the test pixel, performing a subsequent image processing functionusing an offset portion of the shift registers from those used to testthe test pixel.
 13. The method according to claim 12, wherein thesubsequent image processing function is a color processing function. 14.The method according to claim 12, wherein the subsequent imageprocessing function is a color interpolation function.
 15. The methodaccording to claim 12, wherein the subsequent image processing functionis a sharpness filtering function.
 16. The method according to claim 12,wherein the subsequent image processing function is a white balancingfunction.